Steel sheet excellent in fine blanking performance and manufacturing method of the same

ABSTRACT

A steel sheet excellent in FB performance and also excellent in fabrication performance after FB working and a manufacturing method of the same are provided. The steel sheet is a steel sheet having a composition containing from 0.1 to 0.5% of C, not more than 0.5% of Si and from 0.2 to 1.5% of Mn in terms of % by mass, with P and S being adjusted at proper ranges and having a structure having a ferrite having an average grain size of more than 10 μm and less than 20 μm and a cementite present in the ferrite grain having an average particle size of from 0.3 to 1.5 μm. In this way, the steel sheet becomes a steel sheet excellent in FB performance, mold life and performance (side bend elongation) after FB working.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2007/051843, with an international filing date of Jan. 29, 2007 (WO 2007/116599 A1, published Oct. 18, 2007), which is based on Japanese Patent Application No. 2006-100794, filed Mar. 31, 2006.

TECHNICAL FIELD

This disclosure is concerned with steel sheets suitable for applications to automobile parts or the like and, in particular, relates to steel sheets excellent in fine blanking performance suitable for the uses to which fine blanking working (hereinafter also referred to as “FB working”) is applied.

BACKGROUND

In manufacturing complicated mechanical parts, from the viewpoints of an improvement in dimension precision, a reduction in manufacturing process and the like, it is known that fine blanking working is an extremely advantageous working method as compared with machining working.

In usual blanking working, a tool-to-tool clearance is from approximately 5 to 10% of a thickness of a metal sheet as a material to be blanked. However, the fine blanking working differs from the usual blanking working and is a blanking working method of not only setting up the tool-to-tool clearance extremely small as substantially zero (actually, not more than approximately 2% of the thickness of the metal sheet as a material to be blanked) but also making a compression stress act on a material in the vicinity of a tool cutting blade. Then, the fine blanking working has the following characteristic features:

-   -   (1) The generation of a crack from the tool cutting blade is         inhibited, and a fracture surface seen in usual blanking working         becomes substantially zero, whereby a smooth worked surface         (blanked end surface) in which the worked surface is a         substantially 100% shear surface is obtained.     -   (2) The dimensional precision is good.     -   (3) A complicated shape can be blanked by one process.

However, in the fine blanking working, a working ratio which the material (metal sheet) receives is extremely severe. Also, in the fine blanking working, since the working is carried out at a tool-to-tool clearance of substantially zero, there is involved a problem that a load to a mold becomes excessive so that a mold life is shortened. For that reason, materials to which the fine blanking working is applied are required to not only have excellent fine blanking performance but also prevent a reduction in mold life.

In response to these requirements, for example, JP-A-2000-265240 proposes a high carbon steel sheet excellent in fine blanking performance, which has a composition containing from 0.15 to 0.90% by weight of C, not more than 0.4% by weight of Si and from 0.3 to 1.0% by weight of Mn, has a microstructure with a cementite having a spheroidization ratio of 80% or more and an average grain size of from 0.4 to 1.0 μm scattered in a ferrite matrix and has a notch tensile elongation of 20% or more. According to a technology described in JP-A-2000-265240, it is described that the fine blanking performance is improved and that the mold life is also improved.

However, the high carbon steel sheet described in JP-A-2000-265240 involved a problem that fabrication performance after the fine blanking working is inferior.

Also, JP-A-59-76861 proposes a steel sheet for fine blanking prepared by applying proper hot rolling to a billet containing from 0.08 to 0.19% of C and proper amounts of Si, Mn and Al and containing from 0.05 to 0.80% of C and from 0.0005 to 0.005% of B into a steel sheet. It is described that the steel sheet described in JP-A-59-76861 is a steel sheet which is low in a yield strength, high in an impact value, excellent in fine blanking performance, high in an n-value in a low strain region, excellent in combined formability and excellent in quenching property at short-time rapid heating. However, JP-A-59-76861 does not show concrete evaluation regarding the fine blanking performance. Also, the steel sheet described in JP-A-59-76861 involved a problem that fabrication performance after the fine blanking working is inferior.

Also, JP-A-2001-140037 proposes a high carbon steel sheet excellent in flow forming and fine blanking working, which has a composition containing from 0.15 to 0.45% of C, with the contents of Si, Mn, P, S, Al and N being adjusted at proper ranges and has a structure having a fractional ratio of (pearlite+cementite) of not more than 10% and an average grain size of ferrite grain of from 10 to 20 μm. It is described that the high carbon steel sheet described in JP-A-2001-140037 is excellent in fine blanking performance and is improved in mold life in the fine blanking working. However, the high carbon steel sheet described in JP-A-2001-140037 involved a problem that fabrication performance after the fine blanking working is inferior.

Furthermore, it is hard to say that all of the steel sheets described in JP-A-2000-265240, JP-A-59-76861 and JP-A-2001-140037 are not provided with satisfactory and thorough fine blanking performance in the fine blanking working under a recent severe working condition. Also, problems that the mold life is not thoroughly improved and that fabrication performance after the fine blanking working is inferior still remained.

At the beginning, the fine blanking working had been applied to parts to which working is not applied after fine blanking working even among gear parts and the like. However, recently, the application of fine blanking working to automobile parts (for example, reclining parts) tends to expand, and its application to parts which require stretch flanging working, bulging, etc. is investigated. For that reason, steel sheets which are not only excellent in fine blanking performance but also excellent in fabrication performance after fin blanking working in stretch flanging working, bulging, etc. are eagerly desired as automobile parts.

As a technology for improving stretch flanging workability, there have been made a number of proposals up to date. For example, JP-A-9-49065 proposes a wear resistant hot rolled steel sheet excellent in stretch flanging property, which has a composition containing from 0.20 to 0.33% of C, with the contents of Si, Mn, P, S, sol. Al and N being adjusted at proper ranges and further containing from 0.15 to 0.7% of Cr and has a ferrite-bainite mixed structure which may contain pearlite. In the hot rolled steel sheet described in JP-A-9-49065, it is described that by taking the foregoing structure, a hole expansion ratio becomes high, whereby the stretch flanging property is improved. Also, JP-A-2001-214234 proposes a high carbon steel sheet excellent in stretch flanging property, which has a composition containing from 0.2 to 0.7% of C and has a structure in which a cementite average particle size is 0.1 μm or more and less than 1.2 μm and a volume ratio of a cementite-free ferrite grain is not more than 15%. In the high carbon steel sheet described in JP-A-2001-214234, it is described that the generation of a void on an end surface at the time of blanking is inhibited, that the growth of cracks in hole expansion working can be made slow and that the stretch flanging property is improved.

Also, JP-A-9-316595 proposes a high carbon steel sheet excellent in blanking performance and quenching property, which has a composition containing 0.2% or more of C and has a structure composed mainly of ferrite and a cementite and having a cementite particle size of not more than 0.2 μm and a ferrite grain size of from 0.5 to 1 μm. It is described that, according to this, both blanking performance and quenching property which are determined by a burr height and mold life are improved.

The technologies described in JP-A-9-49065 and JP-A-2001-214234 are those made on the assumption that the conventional blanking working is applied, but not those made taking into consideration the application of fine blanking working in which the clearance is substantially zero. Accordingly, it is difficult to ensure similar stretch flanging property after the severe fine blanking working, and even when the stretch flanging property can be ensured, there is encountered a problem that the mold life is short.

Also, in the technology described in JP-A-9-316595, it is necessary that the ferrite grain size is in the range of from 0.5 to 1 μm; and it is difficult to stably manufacture a steel sheet having such a ferrite grain size on an industrial scale, resulting in a problem that the product yield is reduced.

In view of the foregoing problems of the conventional technologies, it could be advantageous to provide steel sheets excellent in fine blanking performance and also excellent in fabrication performance after fine blanking working and manufacturing methods of the same.

SUMMARY

We provide:

-   -   (1) A steel sheet excellent in fine blanking performance, which         is characterized by having a composition containing from 0.1 to         0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not         more than 0.03% of P and not more than 0.02% of S in terms of %         by mass, with the remainder being Fe and unavoidable impurities         and having a structure mainly composed of ferrite and         cementites, wherein the foregoing ferrite has an average grain         size of exceeding 10 μm and less than 20 μm, and of the         foregoing cementites, a cementite present in the ferrite grain         has an average particle size of from 0.3 to 1.5 μm.     -   (2) The steel sheet as set forth in (1), which is characterized         in that in addition to the foregoing composition, the         composition further contains not more than 0.1% of Al in terms         of % by mass.     -   (3) . The steel sheet as set forth in (1) or (2), which is         characterized in that in addition to the foregoing composition,         the composition further contains one or two or more members         selected from not more than 3.5% of Cr, not more than 0.7% of         Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from         0.0005 to 0.005% of B in terms of % by mass.     -   (4) A manufacturing method of a steel sheet excellent in fine         blanking performance including successively applying hot rolling         by heating and rolling a raw steel material to form a hot rolled         sheet and hot rolled sheet annealing by applying annealing to         the subject hot rolled sheet, which is characterized in that the         foregoing raw steel material is a raw steel material having a         composition containing from 0.1 to 0.5% of C, not more than 0.5%         of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P and not         more than 0.02% of S in terms of % by mass, with the remainder         being Fe and unavoidable impurities; the foregoing hot rolling         is a treatment in which a total reduction ratio in a temperature         region of from 800 to 950° C. in finish rolling is set up at 25%         or more, a termination temperature of finish rolling is set up         at from 800 to 950° C., after completion of the subject finish         rolling, cooling is carried out at an average cooling rate of         50° C./s or more and less than 120° C./s, the subject cooling is         stopped at a temperature in the range of from 500 to 700° C.,         and coiling is carried out at from 450 to 600° C.; and the         foregoing hot rolled sheet annealing is carried out at an         annealing temperature of from 600 to 720° C.     -   (5) The manufacturing method of a steel sheet as set forth in         (4), which is characterized in that, in addition to the         foregoing composition, the composition further contains not more         than 0.1% of Al in terms of % by mass.     -   (6) The manufacturing method of a steel sheet as set forth         in (4) or (5), which is characterized in that, in addition to         the foregoing composition, the composition further contains one         or two or more members selected from not more than 3.5% of Cr,         not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to         0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.

A steel sheet which is not only excellent in FB performance but also excellent in performance (side bend elongation) after FB working can be easily and cheaply manufactured, thereby giving rise to remarkable effects in view of the industry. Also, there are brought effects that a steel sheet excellent in FB performance is provided; an end surface treatment after FB working is not necessary; the time of completion of manufacture can be shortened; the productivity is improved; and the manufacturing costs can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph to show a relationship between a ferrite average grain size and a side bend elongation after FB working.

FIG. 2 is a graph to show a relationship between FB performance (average surface roughness on blanked surface: Rz ave) and a cementite average particle size in ferrite grain.

FIG. 3 is an explanatory view to schematically show a measurement region of surface roughness on a blanked surface after FB working.

DETAILED DESCRIPTION

We conducted extensive and intensive investigations regarding influences of a metallographic structure against fine blanking performance (hereinafter abbreviated as “FB performance”), especially influences against morphology and distribution state of ferrite and cementite.

As a result, we found that the FB performance, the fabrication performance after FB working and the mold life are closely related with a particle size of cementite present in a ferrite grain and a ferrite grain size. Then, we found that, when a raw steel material having a composition of a prescribed range is formed into a hot rolled steel sheet having a substantially 100% pearlite structure by making a finish rolling condition of hot rolling and a condition of subsequent cooling proper, which is ten subjected to hot rolling annealing under a proper condition, thereby converting the metallographic structure into a (ferrite+cementite) (granular cementite) structure having a ferrite average grain size of more than 10 μm and less than 20 μm and a cementite average particle size in ferrite grain of from 0.3 to 1.5 μm, the FB performance, the mold life and the fabrication performance (side bend elongation) after FB working are remarkably improved.

In FB working, the material is worked in a state of zero clearance and compression stress. For that reason, there may be a possibility that the material is largely deformed, thereby generating cracks during the subject deformation. When cracks are generated, a fracture surface appears on the blanked surface. To prevent generation of cracks, spheroidization of cementite is important. However, in the case where the cementite is coarsely present in the ferrite grain, on the assumption that a void is easy to be formed between cementite grains at the time of large deformation and that the generation of cracks due to the void growth is unavoidable, we examined the cementite particle size in ferrite grain and the FB performance. Also, with respect to the mold life, we assumed that, when fine cementite is present in the ferrite grain, wear of a cutting blade is accelerated, leading to a reduction in mold life. Furthermore, in the case where fabrication is applied after FB working, we thought that cracks generated at the time of the FB working are connected to each other, leading to a reduction of the molding performance.

First, our experimental results are described.

A high steel slab (corresponding to S35C) containing 0.34% of C, 0.2% of Si and 0.8% of Mn in terms of % by mass was heated at 1150° C. and then subjected to hot rolling consisting of rough rolling of 5 passes and finish rolling of 7 passes, thereby preparing a hot rolled steel sheet having a thickness of 4.2 mm. A total reduction ration in the finish rolling of hot rolling was changed to 10 to 40%; a rolling termination temperature was set up at 860° C.; a coiling temperature was set up at 600° C.; and after the finish rolling, the steel sheet was cooled while changing a cooling rate from 5° C./s to 250° C./s. In the case where cooling (forced cooling) other than air cooling was carried out, a cooling stopping temperature was set up at 650° C. Subsequently, the hot rolled steel sheet was subjected to pickling and then to batch annealing (720° C.×40 h) as hot rolled sheet annealing.

A metallurgical structure of the steel sheet to which hot rolled sheet annealing had been applied was first observed.

In the observation of the metallurgical structure, a specimen was collected from the obtained steel sheet; a cross section parallel to a rolling direction of the subject specimen was polished and corroded with natal; and with respect to a position of ¼ of the sheet thickness, the metallurgical structure was observed by a scanning electron microscope (SEM) and imaged, thereby measuring a ferrite grain size and a cementite particle size in ferrite grain.

The ferrite grain size and the cementite particle size in ferrite grain were quantified by image analysis processing by using “Image Pro Plus ver. 4.0,” which is an image analysis software manufactured by Media Cybernetics, Inc. With respect to the ferrite grain size, an area of each ferrite grain was measured, and a circle-corresponding size was determined from the resulting area and defined as a grain size of each ferrite grain. The thus obtained respective ferrite grain sizes were arithmetically averaged, and its value was defined as a ferrite average grain size of that steel sheet.

Also, in the imaged structure, cementite present on the ferrite grain boundary and cementite present in the ferrite grain were discriminated from each other by means of image analysis; with respect to each cementite grain present in the ferrite grain, a diameter pass through two points on the periphery of the cementite grain and a center of gravity of a corresponding oval of the cementite (an oval having the same area as the cementite and having a primary moment and a secondary moment equal to each other) was measured at every 2° to determine a circle-corresponding size, thereby defining it as a grain size of each cementite grain. The thus obtained respective cementite particle sizes were arithmetically averaged, and its value was defined as a cementite average particle size of that steel sheet. The number of particles of the measured cementite was 3,000 for each.

Also, a specimen (size: 100×80 mm) was collected from the obtained steel sheet and subjected to an FB test. The FB test was carried out by blanking a sample having a size of 60 mm×40 mm (corner radius R: 10 mm) from the specimen by using a 110t hydraulic press machine under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheet thickness) and a working pressure of 8.5 tons. With respect to an end surface (blanked surface) of the blanked sample, a surface roughness (ten-point average roughness Rz) was measured, thereby evaluating the FB performance. With respect to the specimen, to eliminate influences of a deviation in sheet thickness against the clearance, both surfaces were equally ground in advance, thereby regulating the sheet thickness at 4.0±0.010 mm.

With respect to the measurement of the surface roughness, as illustrated in FIG. 3, in each of four end surfaces (sheet thickness surfaces) other than R parts, a region within a range of from 0.5 mm to 3.9 mm of the surface in the punch side in the sheet thickness direction and 10 mm in parallel to the surface (X direction) was scanned 35 times at a pitch of 100 μm in the sheet thickness direction (t direction) by using a contact probe profilometer, and a surface roughness Rz in each scanning line was measured according to JIS B 0601-1994. Furthermore, with respect to the surface roughness Rz on the measured surface, Rzs in the respective scanning lines were summed up, and an average value thereof was employed. The four end surfaces were measured in the same method as described above, and an average surface roughness Rz ave (μm) defined according to the following expression: Rz ave=(Rz 1+Rz 2+Rz 3+Rz 4)/4 (wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface) was computed.

In general, the case where the appearance of the fracture surface on the blanked surface is not more than 10% is defined as “excellent in FB performances.” However, the case where the average surface roughness Rz ave is small as 10 μm or less was defined as “excellent in FB performance.”

Also, the life of the used tool (mold) was evaluated. A surface roughness (ten-point average roughness Rz) of the sample end surface (blanked surface) at the point of time when the number of blanking in FB working reached 30,000 times was measured in the same manner as described above, thereby evaluating the mold life.

Also, a specimen (size: 40 mm×170 mm (rolling direction)) was blanked from the obtained steel sheet by FB working and subjected to a side bend test, thereby evaluating the performance (side bend elongation) after FB working. FB working was carried out under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheet thickness) and a working pressure of 8.5 tons.

A side bend test was carried out in a state of restraining a side surface (sheet surface) of the specimen according to a method of Nagai, et al. (Yoshinori Nagai and Yasutomo Nagai, PK Giho (Press Technical Report), No. 6 (1995), page 14), the subject matter of which is incorporated herein by reference, thereby measuring an elongation at the time of through thickness cracking. An end surface of the specimen in the side of evaluating the elongation was an FB worked surface in the side of a length of 170 mm. Gauge marks for evaluating the elongation at the fracture were written on the specimen by marking-off lines with a gauge mark-to-gauge mark distance of 50 mm. The number of test was two of each steel sheet, and an average value of the obtained elongation values was defined as the side bend elongation value.

The ferrite average grain size and the cementite average particle size in ferrite grain varied depending upon the total reduction ration in the finish rolling of hot rolling and the average cooling rate after the finish rolling. The obtained results are shown in FIGS. 1 and 2.

FIG. 1 shows the relationship between the ferrite average grain size and the side bend elongation. It is noted from FIG. 1 that, when the ferrite average grain size exceeds 10 μm, the side bend elongation exceeds 45% and exhibits a very satisfactory value, and satisfactory performance after FB working is revealed. When the ferrite average grain size is 20 μm or more, burrs after the FB working became large, and the FB performance was reduced. Also, FIG. 2 shows a relationship between the cementite average particle size in ferrite grain and the average surface roughness Rz ave on the blanked surface of FB working in the case where the ferrite average grain size is more than 10 μm and less than 20 μm. It is noted from FIG. 2 that, when the cementite average particle size in ferrite grain is not more than 1.5 μm, Rz ave is not more than 10 μm, and satisfactory FB performance is revealed. When the cementite average particle size in ferrite grain is less than 0.3 μm, the average surface roughness on the blanked surface after blanking 30,000 times exceeded 10 μm, and the mold life was reduced.

The reasons why the composition of our steel sheets are selected are described. The “% by mass” in the composition is expressed merely as “%” unless otherwise indicated.

C: from 0.1 to 0.5%

C is an element influencing the hardness after hot rolling annealing and quenching, and C is required to be contained in an amount of 0.1% or more. When the content of C is less than 0.1%, the hardness required as automobile parts cannot be obtained. On the other hand, since C is contained in a large amount exceeding 0.5%, the steel sheet becomes hard, an industrially sufficient mold life cannot be ensured. For that reason, the content of C is limited to the range of from 0.1 to 0.5%.

Si: not more than 0.5%

Si is an element not only acting as a deoxidizing agent, but also increasing the strength (hardness) due to solution hardening. However, when Si is contained in a large amount exceeding 0.5%, ferrite becomes hard, thereby reducing the FB performance. Also, when Si is contained in an amount exceeding 0.5%, a surface defect known as red scale is generated at the hot rolling stage. For that reason, the content of Si is limited to not more than 0.5%. The content of Si is preferably not more than 0.35%.

Mn: from 0.2 to 1.5%

Mn is an element not only increasing the strength of steel due to solution hardening, but also acting effectively in improving the quenching property. To obtain such an effect, it is desirable that Mn is contained in an amount of 0.2% or more. However, when Mn is contained excessively in an amount exceeding 1.5%, the solution hardening becomes excessively strong so that the ferrite becomes hard, thereby reducing the FB performance. For that reason, the content of Mn is limited to the range of from 0.2 to 1.5%. The content of Mn is preferably from 0.2 to 1.0%, and more preferably from 0.6 to 0.9%.

P: not more than 0.03%

Since P segregates on the grain boundary or the like and reduces performance, it is desirable that P is reduced as far as possible. However, the content of P of up to 0.03% is tolerable. For such a reason, the content of P is limited to not more than 0.03%. The content of P is preferably not more than 0.02%.

S: not more than 0.02%

S is an element which forms a sulfide such as MnS and exists as an inclusion in the steel, thereby reducing the FB performance, and it is desirable that S is reduced as far as possible. However, the content of S of up to 0.02% is tolerable. For such a reason, the content of S is limited to not more than 0.02%. The content of S is preferably not more than 0.01%.

The foregoing components are a basic composition. However, in addition to the foregoing basic composition, Al and/or one or two or more members selected from Cr, Mo, Ni, Ti and B can be contained.

Al: not more than 0.1%

Al is an element not only acting as a deoxidizing agent, but also binding with N to form AlN, thereby contributing to prevention of an austenite grain from coarseness. When Al is contained together with B, Al also has an affect for fixing N, thereby preventing a reduction of the content of B effective for improving the quenching property. Such effects become remarkable when the content of Al is 0.02% or more. However, when the content of Al exceeds 0.1%, the index of cleanliness of steel is reduced. For that reason, when Al is contained, it is preferable that the content of Al is limited to not more than 0.1%. The content of Al as an unavoidable impurity is not more than 0.01%.

All of Cr, Mo, Ni, Ti and B are elements contributing to an improvement in quenching property and/or an improvement in resistance to temper softening and can be selected and contained as the need arises.

Cr: not more than 3.5%

Cr is an element effective for improving the quenching property. To obtain such an effect, it is preferable that Cr is contained in an amount of 0.1% or more. However, when the content of Cr exceeds 3.5%, not only the FB performance is reduced, but also an excessive increase of the resistance to temper softening is brought about. For that reason, when Cr is contained, it is preferable that the content of Cr is limited to not more than 3.5%. The content of Cr is more preferably from 0.2 to 1.5%.

Mo: not more than 0.7%

Mo is an element acting to effectively improve the quenching property. To obtain such an effect, it is preferable that Mo is contained in an amount of 0.05% or more. However, when the content of Mo exceeds 0.7%, the steel becomes hard, thereby reducing the FB performance. For that reason, when Mo is contained, it is preferable that the content of Mo is limited to not more than 0.7%. The content of Mo is more preferably from 0.1 to 0.3%.

Ni: not More than 3.5%

Ni is an element effective for improving the quenching property. To obtain such an effect, it is preferable that Ni is contained in an amount of 0.1% or more. However, when the content of Ni exceeds 3.5%, the steel becomes hard, thereby reducing the FB performance. For that reason, when Ni is contained, it is preferable that the content of Ni is limited to not more than 3.5%. The content of Ni is more preferably from 0.1 to 2.0%.

Ti: from 0.01 to 0.1%

Ti is easy to bind with N to form TiN and is an element effectively acting to prevent coarseness of a γ grain at the time of quenching. Also, when Ti is contained together with B, since Ti reduces N which forms BN, it has an effect of minimizing the addition amount of B necessary for improving the quenching property. To obtain such effects, it is required that the content of Ti is 0.01% or more. On the other hand, when the content of Ti exceeds 0.1%, the ferrite is subjected to precipitation strengthening due to precipitation of TiC or the like and becomes hard, thereby reducing the mold life. For that reason, when T is contained, it is preferable that the content of Ti is limited to the range of from 0.01 to 0.1%. The content of Ti is more preferably from 0.015 to 0.08%.

B: from 0.0005 to 0.005%

B is an element which segregates on an austenite grain boundary and when contained in a trace amount, improves the quenching property. In particular, the case where B is compositely added together with Ti is effective. To improve the quenching property, it is required that the content of B is 0.0005% or more. On the other hand, even when B is contained in an amount exceeding 0.005%, the effect is saturated and an effect that corresponds to the content cannot be expected, and therefore, such is economically disadvantageous. For that reason, when B is contained, it is preferable that the content of B is limited to the range of from 0.0005 to 0.005%. The content of B is more preferably from 0.0008 to 0.004%.

The remainder other than the foregoing components is Fe and unavoidable impurities. As unavoidable impurities, for example, not more than 0.01% of N, not more than 0.01% of 0 and not more than 0.1% of Cu are tolerable.

Next, the reasons why the structure of our steel sheets are selected are described.

The steel sheets have a structure composed mainly of ferrite and cementite. The “structure composed mainly of ferrite and cementite” as referred to herein means a structure in which ferrite and cementite account for 95% or more in terms of a volume ratio. That is, though the steel sheet has a composition made of ferrite and cementite, other phases than the ferrite and cementite can be tolerated in an amount of up to approximately 5% in terms of a volume ratio.

In the steel sheet, a grain size of the ferrite is more than 10 μm and less than 20 μm in terms of an average grain size. When the ferrite average grain size is not more than 10 μm, the side bend elongation after FB working is reduced as shown in FIG. 1. While this reason is not elucidated, we believe that, when the ferrite grain size is not more than 10 μm, since a diffusion rate is fast on the ferrite grain boundary and an average particle size of the cementite present on the ferrite grain boundary is easy to become large, a void is generated between the cementite grains on the ferrite grain boundary due to large deformation at the time of FB working and grows, thereby easily forming cracks; and that the cracks develop at the time of fabrication after FB working and are united, whereby the side bend elongation after FB working is reduced. On the other hand, when the ferrite average grain size is 20 μm or more, though the steel sheet is softened so that the mold life is improved, a burr height after working remarkably increases. For that reason, the ferrite average grain size is limited to more than 10 μm and less than 20 μm. It is preferably from 12 to 18 μm.

Also, in the steel sheet, of the cementite grains, cementite present in the ferrite grain has an average particle size in the range of from 0.3 to 1.5 μm. When the average particle size of cementite present in the ferrite grain is less than 0.3 μm, the steel plate becomes hard, whereby the mold life is reduced. On the other hand, when the cementite particle exceeds 1.5 μm and becomes coarse, as shown in FIG. 2, a void is generated between the cementite grains due to large deformation at the time of FB working and grows to form cracks, fracture surfaces are generated so that the roughness of the worked surface (blanked surface) increases, and the FB performance is reduced. For that reason, the cementite average particle size in ferrite grain is limited to the range of from 0.3 to 1.5 μm.

Next, preferred manufacturing methods of the steel sheets are described.

It is preferable that a molten steel having the foregoing composition is molten by a common melting method using a converter or the like and formed into a raw steel material (slab) by a common casting method such as a continuous casting method.

Subsequently, the obtained raw steel material is subjected to hot rolling to form a hot rolled sheet by heating and rolling.

The hot rolling is a treatment in which a total reduction ratio in a temperature region of from 800 to 950° C. in finish rolling is set up at 25% or more, a termination temperature of finish rolling is set up at from 800 to 950° C., after completion of the finish rolling, cooling is carried out at an average cooling rate of 50° C./s or more and less than 120° C./s, the subject cooling is stopped at a temperature in the range of from 500 to 700° C., and coiling is carried out at from 450 to 600° C.

In the hot rolling, by adjusting the termination temperature of finish rolling and the subsequent cooling condition, a hot rolled steel sheet having a substantially 100% pearlite structure is obtained. In addition, in the hot rolling, by regulating the total reduction ratio in a temperature region of from 800 to 950° C. in finish rolling at 25% or more, after proper hot rolled sheet annealing, a structure having a ferrite average grain size of more than 10 μm and less than 20 μm is obtained.

Total Reduction Ratio in Temperature Region of from 800 to 950° C. in Finish Rolling: 25% or More

In the finish rolling of hot rolling, by increasing the reduction ratio, the austenite grain size becomes small; following this, the pearlite grain size after transformation becomes fine; and in the hot rolled sheet annealing, the growth of the ferrite grain is accelerated while applying, as a driving force, high grain boundary energy that the fine pearlite possesses.

In particular, at high temperatures exceeding 950° C., since the austenite grain size is easy to become large due to recrystallization, influences of the rolling in a temperature region of not higher than 950° C. are large.

The pearlite changes into polygonal ferrite and spherical cementite due to hot rolled sheet annealing. To make the ferrite formed by this hot rolled sheet annealing have an average grain size of more than 10 μm and less than 20 μm, the total reduction ratio in a temperature region of from 800 to 950° C. in finish rolling is 25% or more, a value of which is a reduction ratio larger than that in usually performed rolling. When the total reduction ratio in a temperature of from 800 to 950° C. is less than 25%, the reduction ratio is insufficient, and it becomes too difficult to make the ferrite grain size fall within a desired range. From the viewpoint of a rolling load, it is preferable that an upper limit of the total reduction ratio is not more than 35%. The total reduction ratio is more preferably from 25 to 33%.

Termination Temperature of Finish Rolling: from 800 to 950° C.

When the termination temperature of finish rolling exceeds 950° C. and becomes high, not only a generated scale becomes thick so that the pickling property is reduced, but also a decarburized layer may possibly be formed in the steel sheet surface layer, whereby the ferrite grain size is easy to become coarse. On the other hand, when the termination temperature of finish rolling is lower than 800° C., an increase in the rolling load becomes remarkable, and an excessive load against a rolling mill becomes problematic. For that reason, it is preferable that the termination temperature of finish rolling is a temperature in the range of from 800 to 950° C.

Average Cooling Rate after Completion of Finish Rolling: 50° C./s or More and Less than 120° C./s

After completion of the finish rolling, cooling is carried out at an average cooling rate of 50° C./s or more. The subject average cooling rate is an average cooling rate of from the termination temperature of finish rolling to a stopping temperature of the subject cooling (forced cooling). When the average cooling rate is less than 50° C./s, cementite-free ferrite is formed during cooling, and the structure after cooling is a heterogeneous structure of (ferrite+pearlite), whereby a homogeneous structure composed of substantially 100% pearlite cannot be ensured. When the hot rolled sheet structure is a heterogeneous structure of (ferrite+pearlite), the cementite distribution is also heterogeneous, and whatever the subsequent hot rolled sheet annealing is devised, the cementite present in the grain easily becomes coarse. Accordingly, it is preferable that the average cooling rate after completion of the finish rolling is limited to 50° C./s or more. From the viewpoint of preventing the formation of bainite, it is preferable that the average cooling rate after completion of the finish rolling is less than 120° C./s. When the average cooling rate is 120° C./s or more, since the structure is easy to differ between the steel sheet surface layer part and the sheet thickness central part and after the hot rolled sheet annealing, deformability differs between the surface layer part and the sheet thickness central part, the mold lifer, the FB performance and the fabrication performance after FB working are easily reduced. For that reason, it is preferable that the average cooling rate after the finish rolling is 50° C./s or more and less than 120° C./s.

Cooling Stopping Temperature: from 500 to 700° C.

It is preferable that a temperature at which the foregoing cooling (forced cooling) is stopped is from 500 to 700° C. When the cooling stopping temperature is lower than 500° C., there are caused problems in operation such as a problem that hard bainite or martensite is formed, whereby the hot rolled sheet annealing takes a long time; and generation of cracks during coiling. On the other hand, when the cooling stopping temperature exceeds 700° C. and becomes high, since a ferrite transformation noise is present in the vicinity of 700° C., ferrite is formed during cooling after stopping of cooling, whereby a homogeneous structure composed of substantially 100% pearlite cannot be ensured. From these matters, it is preferable that the cooling stopping temperature is limited to a temperature in the range of form 500 to 700° C. The cooling stopping temperature is more preferably from 500 to 650° C., and further preferably from 500 to 600° C.

After stopping the cooling, the hot rolled sheet is immediately coiled in a coil state.

Coiling Temperature: from 450 to 600° C.

When the coiling temperature is lower than 450° C., cracks form in the steel sheet at the time of coiling, resulting in a problem in operation. On the other hand, when the coiling temperature exceeds 600° C., there is a problem that ferrite is formed during the coiling.

The coiling temperature is preferably from 500 to 600° C.

The thus obtained hot rolled sheet (hot rolled steel sheet) is then subjected to removal of an oxidized scale of the surface by pickling or shot blasting and subsequently to hot rolled sheet annealing at an annealing temperature of from 600 to 720° C. By applying proper hot rolled sheet annealing to the hot rolled sheet having a substantially 100% pearlite structure, spheroidization of cementite is accelerated, whereby not only the ferrite grain size is adjusted at a desired range, but also the cementite particle size in ferrite grain can be adjusted at a desired range.

Annealing Temperature of Hot Rolled Sheet Annealing: from 600 to 720° C.

When the annealing temperature is lower than 600° C., the cementite average particle size in ferrite grain is less than 0.3 μm. On the other hand, the annealing temperature exceeds 720° C. and becomes high, the cementite particle size in ferrite grain exceeds 1.5 μm, and the FB performance is reduced. Though a holding time of the hot rolled sheet annealing is not required to be particularly limited, to adjust the cementite particle range at a desired range, it is preferable that the holding time is 8 hours or more. Also, when it exceeds 80 hours, since the ferrite grain becomes excessively coarse and the cementite average particle size in ferrite grain may possibly exceed 1.5 μm, the holding time is preferably not more than 80 hours.

Examples

A raw steel material (slab) having a composition shown in Table 1 was used as a starting material. Such a raw material was heated at a heating temperature shown in Table 2, and a hot rolled sheet having a thickness of 4.2 mm was then prepared under a hot rolling condition shown in Table 2.

With respect to the hot rolling condition, the total reduction ratio in a temperature region of from 800° C. to 950° C. in finish rolling, the rolling termination temperature of finish rolling, the average cooling rate in cooling after completion of the finish rolling, the cooling stopping temperature and the coiling temperature were varied.

Such a hot rolled sheet was then subjected to batch annealing and pickling. The obtained steel sheet was evaluated with respect to structure observation, FB performance and performance (side bend elongation) after FB working. The test methods are as follows.

(1) Structure Observation:

A specimen for structure observation was collected from the obtained steel sheet. A cross section parallel to a rolling direction of the specimen was polished and corroded with nital; and with respect to a position of ¼ of the sheet thickness, a metallurgical structure was observed (field number: 30 places) by a scanning electron microscope (SEM) (magnification, ferrite: 1,000 times, cementite: 3,000 times); a volume ratio of ferrite and a cementite, a ferrite grain size and a cementite particle size in ferrite grain were measured by image analysis processing by using “Image Pro Plus ver. 4.0,” which is an image analysis software manufactured by Media Cybernetics, Inc.

With respect to the volume ratio of ferrite and cementite, the metallurgical structure was observed (field number: 30 places) by SEM (magnification: 3,000 times); the area ratio obtained by dividing an area resulting from summing up an area of ferrite excluding cementite and an area of cementite by a total field area; and this value was judged as a volume ratio of ferrite and cementite. With respect to the ferrite grain size, an area of each ferrite grain was measured, and a circle-corresponding size was determined from the resulting area and defined as a grain size of each ferrite grain. The thus obtained respective ferrite grain sizes were arithmetically averaged, and its value was defined as a ferrite average grain size of that steel sheet. The measured area ratio was determined with respective to 500 grains for each.

With respect to the cementite particle size in ferrite grain, cementite present in the ferrite grain was discriminated in each field (field number: 30 places) of the observation of a metallurgical structure (magnification: 3,000 times); with respect to each cementite grain present in the ferrite grain, a diameter passing through two points on the periphery of the cementite grain and a center of gravity of a corresponding oval of the cementite grain (an oval having the same area as the cementite grain and having a primary moment and a secondary moment equal to each other) was measured at every 2° to determine a circle-corresponding size, thereby defining it as a particle size of each cementite grain. The thus obtained respective cementite particle sizes were averaged, and its value was defined as a cementite average particle size in ferrite grain. The number of particles of the measured cementite was 3,000 for each.

(2) FB Performance:

A specimen (size: 100×80 mm) was collected from the obtained steel sheet and subjected to an FB test. The FB test was carried out by blanking a sample having a size of 60 mm×40 mm (corner radius R: 10 mm) from the specimen by using a 110t hydraulic press machine under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheet thickness) and a working pressure of 8.5 tons. With respect to an end surface (blanked surface) of the blanked sample, the surface roughness (ten-point average roughness Rz) was measured, thereby evaluating the FB performance. With respect to the specimen, to eliminate influences of a deviation in sheet thickness against the clearance, both surfaces were equally ground in advance, thereby regulating the sheet thickness at 4.0±0.010 mm.

With respect to the measurement of the surface roughness, as illustrated in FIG. 3, in each of four end surfaces (sheet thickness surfaces) other than R parts, a region within a range of from 0.5 mm to 3.9 mm of the surface in the punch side in the sheet thickness direction and 10 mm in parallel to the surface (X direction) was scanned 35 times at a pitch of 100 μm in the sheet thickness direction (t direction) by using a contact probe profilometer, and a surface roughness Rz in each scanning line was measured according to JIS B0601-1994. Furthermore, with respect to the surface roughness Rz on the measured surface, Rzs in the respective scanning lines were summed up, and an average value thereof was employed. The four end surfaces were measured in the same method as described above, and an average surface roughness Rz ave (μm) defined according to the following expression was computed, thereby evaluating the FB performance:

Rz ave=(Rz 1+Rz 2+Rz 3+Rz 4)/4

(wherein Rz 1, Rz 2, R 3 and Rz 4 each represents Rz on each surface).

As described previously, the case where the average surface roughness Rz ave is small as 10 μm or less was defined as “excellent in FB performance.”

Also, the presence or absence of the generation of a large burr (burr height) which is a problem as FB working was observed.

Also, the life of the used tool (mold) was evaluated. A surface roughness (ten-point average roughness Rz) of the sample end surface (blanked surface) at the point of time when the number of blanking in FB working reached 30,000 times was measured, thereby evaluating the mold life. The measurement method of the surface roughness was the same as described above. The case where the average surface roughness Rz ave of the sample end surface is not more than 10 μm is defined as “◯,” the case where it is more than 10 μm and not more than 16 μm was defined as “Δ,” and the case where it is more than 16 μm was defined as “X.”

(3) Performance (Side Bend Elongation) after FB Working:

A specimen (size: 40 mm×170 mm (rolling direction)) was blanked from the obtained steel sheet by FB working and subjected to a side bend test, thereby evaluating the performance (side bend elongation) after FB working.

To eliminate influences of a deviation in sheet thickness against the clearance, both surfaces were equally ground in advance, thereby regulating the sheet thickness at 4.0±0.010 mm. FB working was carried out under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5% of the sheet thickness) and a working pressure of 8.5 tons.

A side bend test was carried out in a state of restraining a side surface (sheet surface) of the specimen according to a method of Nagai, et al. (Yoshinori Nagai and Yasutomo Nagai, PK Giho (Press Technical Report), No. 6 (1995), page 14), the subject matter of which is incorporated by reference, thereby measuring an elongation at the time of through thickness cracking. An end surface of the specimen in the side of evaluating the elongation was an FB worked surface in the side of a length of 170 mm. Gauge marks for evaluating the elongation at the fracture were written on the specimen by marking-off lines with a gauge mark-to-gauge mark distance of 50 mm. The number of tests was two of each steel sheet, and an average value of the obtained elongation values was defined as the side bend elongation value. The performance (side bend elongation) after the FB working was evaluated such that the case where the side bend elongation value is 45% or more is defined as “◯,” and that the case where it is less than 45% is defined as “X.”

The obtained results are shown in Table 3.

In all of our examples, the average surface roughness Rz ave on the blanked surface is not more than 10 μm; the FB performance is excellent; the blanked surface at the time of 30,000 times in blanking number is smooth (evaluation: ◯); and a reduction in mold life is not acknowledged. Also, our examples are excellent in the side bend elongation (Performance) after FB working. In our examples, it was confirmed that the sum of volume ratio of the ferrite and cementite is 95% or more, thereby forming a structure composed mainly of ferrite and a cementite. On the other hand, in the comparative examples, the average surface roughness Rz ave on the blanked surface exceeds 10 μm and becomes coarse, whereby the FB performance is reduced; or a large burr is generated at the time of FB working; or the mold life is reduced; the side bend elongation (performance) after FB working is reduced; or all of the FB performance, the mold life and the side bend elongation (performance) after FB working are reduced.

TABLE 1 Steel Chemical components (% by mass) No. C Si Mn P S Al N Cr Mo Ni Ti B A 0.15 0.19 0.65 0.014 0.0041 — 0.0066 0.94 0.16 — — — B 0.29 0.03 0.51 0.013 0.0030 — 0.0043 0.23 — — 0.015 0.0024 C 0.35 0.20 0.71 0.013 0.0035 — 0.0029 — — — — — D 0.35 0.02 0.55 0.009 0.0032 — 0.0036 0.21 — — 0.015 0.0024 E 0.49 0.19 0.73 0.011 0.0042 — 0.0032 — — — — — F 0.46 0.19 0.76 0.014 0.0050 — 0.0030 — — — — — G 0.35 0.22 0.72 0.012 0.0037 — 0.0039 0.98 0.17 — — — H 0.17 0.02 0.72 0.021 0.0035 — 0.0033 — — — — — I 0.11 0.02 1.42 0.015 0.0038 — 0.0029 — — — — — J 0.21 0.20 0.71 0.016 0.0045 — 0.0042 1.02 — — — — K 0.19 0.24 0.76 0.015 0.0037 — 0.0039 0.48 0.17 0.46 — — L 0.34 0.21 0.74 0.015 0.0040 0.035 0.0029 — — — — — M 0.23 0.19 0.73 0.013 0.0046 0.028 0.0038 0.80 0.26 1.19 0.02 0.0018 N 0.25 0.23 0.69 0.015 0.0031 — 0.0027 0.91 0.25 — 0.02 0.0024 O 0.22 0.20 0.75 0.014 0.0050 — 0.0033 — — 1.43 — — P 0.19 0.69 0.73 0.016 0.0033 — 0.0040 — — — — — Q 0.36 0.21 1.69 0.013 0.0059 — 0.0048 — — — — —

TABLE 2 Hot rolling condition Total Termination Average Cooling Annealing Steel Heating reduction temperaure cooling stopping Coiling Annealing plate Steel temperature ratio* of finish rate temperature temperature temperature Annealing No. No. (° C.) (%) rolling (° C.) (° C./s) (° C.) (° C.) (° C.) time (hr) Remark 1 A 1150 33 850 115 570 550 720 40 Invention 2 A 1200 27 900 100 660 580 710 40 Invention 3 B 1170 29 865 90 690 590 715 30 Invention 4 B 1220 31 840 105 630 570 710 30 Invention 5 C 1200 28 880 75 550 540 720 40 Invention 6 C 1100 35 850 115 620 550 690 40 Invention 7 D 1210 29 885 110 560 530 720 50 Invention 8 D 1200 27 890 65 680 580 690 30 Invention 9 E 1150 28 855 90 520 470 680 40 Invention 10 E 1200 26 860 75 580 560 710 35 Invention 11 F 1170 27 850 80 550 520 720 60 Invention 12 F 1220 29 835 100 620 490 700 40 Invention 13 G 1150 25 870 115 570 530 720 50 Invention 14 G 1170 28 885 65 500 460 690 40 Invention 15 H 1200 26 925 85 680 580 720 30 Invention 16 H 1210 27 890 110 510 490 680 30 Invention 17 I 1150 28 860 70 620 530 710 50 Invention 18 I 1170 26 880 115 660 510 700 30 Invention 19 J 1200 28 875 95 580 560 720 40 Invention 20 J 1150 26 895 80 680 570 710 30 Invention 21 K 1250 27 910 75 670 580 710 40 Invention 22 K 1150 32 880 100 540 540 700 50 Invention 23 A 1210 38 790 75 530 520 710 30 Comparison 24 A 1200 21 880 95 690 680 700 40 Comparison 25 B 1200 27 880 60 720 710 720 40 Comparison 26 B 1170 26 960 115 610 520 680 40 Comparison 27 C 1210 29 855 15 660 640 710 30 Comparison 28 C 1250 22 870 90 600 565 720 30 Comparison 29 D 1200 28 755 85 620 610 700 50 Comparison 30 D 1190 27 890 150 470 460 715 40 Comparison 31 E 1180 30 835 60 680 560 740 30 Comparison 32 E 1200 31 820 110 600 470 590 30 Comparison 33 F 1200 19 905 80 630 610 720 40 Comparison 34 F 1230 26 885 75 730 720 710 40 Comparison 35 G 1200 35 750 85 550 525 700 30 Comparison 36 G 1180 27 860 135 670 640 720 30 Comparison 37 H 1180 25 905 40 710 670 690 40 Comparison 38 H 1200 27 970 90 570 520 710 30 Comparison 39 I 1210 26 905 85 510 405 — — Comparison** 40 I 1190 17 855 100 600 560 580 40 Comparison 41 J 1190 28 880 70 420 340 — — Comparison** 42 J 1210 29 840 140 710 550 720 40 Comparison 43 K 1170 20 780 115 620 570 700 40 Comparison 44 K 1200 33 780 65 640 605 690 50 Comparison 45 L 1210 27 850 55 680 590 710 30 Invention 46 M 1230 26 880 70 660 490 680 30 Invention 47 N 1200 27 860 85 600 460 720 30 Invention 48 O 1220 26 850 100 510 510 700 40 Invention 49 P 1230 28 880 110 630 600 710 30 Comparison 50 Q 1200 26 890 85 600 590 700 50 Comparison *Total reduction ratio at from 800 to 950° C. in finish rolling; **A crack was generated at the time of coiling.

TABLE 3 Steel Ferrite Cementite average FB performance Performance after FB working plate Steel average grain particle size in Rz ave Mold Side bend Evalu- No. No. size (μm) ferrite grain (μm) (μm) life elongation (%) ation Remark 1 A 16.2 0.56 6 ◯ 50.4 ◯ Invention 2 A 17.9 0.83 8 ◯ 47.6 ◯ Invention 3 B 18.4 0.84 9 ◯ 46.1 ◯ Invention 4 B 16.8 0.68 6 ◯ 48.3 ◯ Invention 5 C 17.3 0.45 5 ◯ 48.5 ◯ Invention 6 C 16.2 0.79 8 ◯ 47.2 ◯ Invention 7 D 16.7 0.59 5 ◯ 49.7 ◯ Invention 8 D 18.1 0.44 10 ◯ 46.7 ◯ Invention 9 E 15.7 0.61 8 ◯ 47.3 ◯ Invention 10 E 12.5 0.32 7 ◯ 47.8 ◯ Invention 11 F 17.4 0.86 6 ◯ 48.6 ◯ Invention 12 F 16.3 0.75 8 ◯ 47.5 ◯ Invention 13 G 14.7 0.80 5 ◯ 52.2 ◯ Invention 14 G 11.2 0.46 8 ◯ 45.3 ◯ Invention 15 H 18.3 0.84 9 ◯ 46.6 ◯ Invention 16 H 14.7 0.63 7 ◯ 49.0 ◯ Invention 17 I 12.9 0.85 6 ◯ 48.4 ◯ Invention 18 I 15.9 0.37 8 ◯ 47.5 ◯ Invention 19 J 14.3 0.66 4 ◯ 49.9 ◯ Invention 20 J 19.6 0.41 9 ◯ 47.1 ◯ Invention 21 K 18.8 0.38 8 ◯ 46.5 ◯ Invention 22 K 16.6 0.70 5 ◯ 51.8 ◯ Invention 23 A 8.4 0.21 16 X 42.9 X Comparison 24 A 9.1 0.41 9 Δ 43.7 X Comparison 25 B 13.4 1.63 19 X 45.2 ◯ Comparison 26 B 22.0 0.57 9 ◯ 47.0 ◯ Comparison*** 27 C 11.5 1.56 17 X 45.5 ◯ Comparison 28 C 7.3 0.44 8 ◯ 38.9 X Comparison 29 D 11.9 1.58 18 X 46.1 ◯ Comparison 30 D 23.7 0.23 19 X 45.5 ◯ Comparison*** 31 E 12.6 1.71 22 X 46.8 ◯ Comparison 32 E 13.5 0.20 24 X 45.3 ◯ Comparison 33 F 7.4 0.26 17 X 39.7 X Comparison 34 F 14.2 1.55 19 X 47.6 ◯ Comparison 35 G 15.3 1.65 17 X 46.4 ◯ Comparison 36 G 25.6 0.22 20 X 45.2 ◯ Comparison*** 37 H 21.0 1.57 16 Δ 46.7 ◯ Comparison*** 38 H 23.2 0.68 8 Δ 46.1 ◯ Comparison*** 39 I — — — — — — Comparison** 40 I 8.8 0.19 18 X 38.2 X Comparison 41 J — — — — — — Comparison** 42 J 27.4 0.23 23 X 45.8 ◯ Comparison*** 43 K 9.3 0.22 21 X 42.9 X Comparison 44 K 13.4 1.60 19 X 46.0 ◯ Comparison 45 L 11.7 0.95 8 ◯ 46.3 ◯ Invention 46 M 13.9 0.40 7 ◯ 48.4 ◯ Invention 47 N 15.4 1.45 7 ◯ 47.5 ◯ Invention 48 O 12.6 1.09 7 ◯ 47.8 ◯ Invention 49 P 13.3 0.72 16 Δ 38.2 X Comparison 50 Q 11.2 0.53 14 Δ 35.7 X Comparison **A Crack was generated at the time of coiling. ***A burr was generated. 

1-6. (canceled)
 7. A steel sheet excellent in fine blanking performance, having a composition comprising from 0.1 to 0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P and not more than 0.02% of S in terms of % by mass, with the remainder being Fe and unavoidable impurities and having a structure mainly composed of ferrite and cementite, wherein said ferrite has an average grain size of exceeding 10 μm and less than 20 μm, and of said cementite, cementite present in the ferrite grain has an average particle size of from 0.3 to 1.5 μm.
 8. The steel sheet according to claim 7, wherein the composition further comprises not more than 0.1% of Al in terms of % by mass.
 9. The steel sheet according to claims 7, wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.
 10. The steel sheet according to claims 8, wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.
 11. A method of manufacturing a steel sheet excellent in fine blanking performance including successively applying hot rolling by heating and rolling a raw steel material to form a hot rolled sheet and hot rolled sheet annealing by applying annealing to the hot rolled sheet, wherein the raw steel material has a composition comprising from 0.1 to 0.5% of C, not more than 0.5% of Si, from 0.2 to 1.5% of Mn, not more than 0.03% of P and not more than 0.02% of S in terms of % by mass, with the remainder being Fe and unavoidable impurities; said hot rolling is a treatment in which a total reduction ratio in a temperature region of from 800 to 950° C. in finish rolling is at 25% or more, a termination temperature of finish rolling is from 800 to 950° C., after completion of the finish rolling, cooling is carried out at an average cooling rate of 50° C./s or more and less than 120° C./s, the cooling is stopped at a temperature from 500 to 700° C., and coiling is carried out at from 450 to 600° C.; and said hot rolled sheet annealing is carried out at an annealing temperature from 600 to 720° C.
 12. The method according to claim 11, wherein the composition further comprises not more than 0.1% of Al in terms of % by mass.
 13. The method according to claim 11, wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass.
 14. The method according to claim 12, wherein the composition further comprises one or more members selected from the group consisting of not more than 3.5% of Cr, not more than 0.7% of Mo, not more than 3.5% of Ni, from 0.01 to 0.1% of Ti and from 0.0005 to 0.005% of B in terms of % by mass. 